Protective Panels

ABSTRACT

A protective panel takes the form of a cement mortar panel made of cement mortar containing metal fibres and, optionally, polymer fibres. A core volume of the panel is reinforced by a network of 3D wire cells. The network of 3D wire cells may be defined by multiple layers of metal mesh that are fixedly connected or integrated together. Such protective panels are capable of resisting fire, explosion and/or blast impact.

The present invention relates to concrete panels, in particular cladding panels having a protective function. The protective panels may be capable of resisting fire, explosion and/or blast impact.

Cladding or lining panels for walls, ceilings, tunnels or other industrial or civil buildings can provide a “defence line” against fire and/or explosions, including terrorist attacks where such risks exist. However, a problem with cladding such structures with protective panels is that the panels should not be too thick or heavy for ease of transportation and installation and to minimise the additional weight on the existing structure. Conventional concrete panels are not generally suitable for protective claddings as they tend to shatter when exposed to blasts due to the material's high brittleness. As concrete is known to be brittle under stresses or impact loads, various proposals have been made to strengthen concrete using metal reinforcement, e.g. in the form of steel bars (so-called rebars). The problem with conventional steel reinforcement is that it does not improve the brittleness of the concrete. This means that the concrete remains at high risk to explosive spalling under fire and to fragmentation under blast effect.

It is also known to improve the mechanical properties of concrete by incorporating a relatively small amount (per unit volume) of steel fibres, for example as is described in U.S. Pat. No. 6,478,867. This ultra-high performance fibre reinforced concrete (UHPFRC) comprises cement having a fibre content of less than 4% of the volume of the concrete after it has set. This is a high-tech concrete prepared from a mix comprising aggregate particles having a maximum size of at most 2 mm and metal fibres of length from 10 to 30 mm. It should be noted that such UHPFRC is not comparable to conventional concretes as it does not contain conventional aggregates (all its mineral components have a size of less than a few millimetres) and it contains small fibres, e.g. of length 13 mm and diameter 0.2 mm. The material has a high manufacturing cost and therefore may be unsuitable for standard use in conventional constructions. Furthermore, such steel fibre reinforced concrete has very poor fire resistance due to the low permeability of the high-strength mortar which makes it more susceptible to explosive spalling during fire. Panels of UHPFRC have been found to perform badly in fire and impact tests (simulating blast load) carried out at the University of Ulster.

Steel-reinforced concrete is known to be unstable when exposed to fire with failure often resulting from explosive spelling. It has been proposed that the fire resistance of concrete panels can be improved by adding polymer fibres to the cement mix. It is thought that when the fibres melt under high temperatures they develop pores or pathways in the cement through which steam or water can escape to avoid the build-up of pressure that is known to result in spalling. However, there has yet to be developed a concrete mix that is particularly suited to making cladding panels that can withstand both fire and blast impacts.

It is an object of the present invention to provide concrete panels having an improved performance.

According to a first aspect the present invention provides a concrete panel made of cement mortar containing metal fibres and polymer fibres, wherein a core volume of the panel is reinforced by one or more layers of metal mesh.

It will be appreciated that such “concrete” panels may be distinguished from conventional panels made of concrete, as concrete requires the presence of coarse aggregate material such as stone or gravel that is discernible to the human eye. A typical concrete mix is cement, water and aggregate (e.g. gravel and sand). Cement mortar, on the other hand, is a mix comprising cement, water and sand (or other fine aggregate). For a concrete material to be classed as “cement mortar” it must contain only sand, or else very fine aggregate e.g. of size less than 5 mm. According to another aspect the present invention may be considered to provide a cement mortar panel containing metal fibres and polymer fibres, wherein a core volume of the panel is reinforced by one or more layers of metal mesh.

It will be understood that concrete i.e. cement mortar panels in accordance with the invention are provided with the three main benefits of incorporating a metal mesh reinforcement for mechanical integrity and to improve the material's ductile response to explosions, polymer fibre reinforcement to protect from spalling in the event of fire, and metal fibres to significantly reinforce the surface layers that normally suffer from spalling during fire and fragmentation under blast effect.

In order to optimise the ductility provided by the layer(s) of metal mesh, it has been found preferable to provide a 3D network of interconnected layers. In a preferred set of embodiments, multiple layers of metal mesh are fixedly connected or integrated together e.g. to define a network of 3D wire cells. The metal mesh is preferably formed of wires. The 3D cells are then defined by the metal wires of the interconnected mesh layers. A metal mesh reinforcement, especially a 3D network of metal wire cells, has been found particularly beneficial in providing a “spring” effect that absorbs shock wave energy induced by a blast. The infrastructure provided by a 3D network of wire cells also has a significant effect in maintaining the integrity of the panel, which increases the fire resistance and prevents explosive spalling when exposed to high temperatures. Such concrete panels can therefore find use in fire-blast protection systems which provide protection for buildings, tunnels, industrial premises (or any other structures) against events of: a) fire only, b) blast only (including terrorist attacks), or c) fire and blast at the same time.

Furthermore, it has been recognised that by confining one or more layers of metal mesh reinforcement, and preferably a network of 3D wire cells, to a core volume of the panel, the bulk metal reinforcement does not reach the surface(s) of the panel where it could result in spalling in the presence of a fire. Instead, the polymer fibre reinforcement in the surface layers ensures that the panel has sufficient fire resistance.

It has been recognised that the layer(s) of metal mesh (preferably defining a network of 3D wire cells) in the core volume of the panel are important for enhancing the strength of the concrete i.e. cement mortar panel and increasing its ductility and elasticity and to shift the concrete panel behaviour so that it performs as a “steel-like” panel. Any kind of metal mesh, i.e. interconnected grid of metal material, can be used. Galvanised steel wire mesh may be readily available. As is mentioned above, it has been found preferable to provide multiple layers of metal mesh. Multiple layers of metal wire mesh or grid of different size and shape openings may be connected together to form a network of 3D cells. Preferably the layers of metal mesh are fixedly connected or integrated together, e.g. by means of point welding or using steel wire fasteners. This helps to ensure that the mesh layers are closely packed together so as to prevent concrete fragmentation under blast forces, to minimise the core volume containing the mesh reinforcement and to reduce the size of the panels for a given strength.

There is a trade-off between increasing the number of layers or thickness of the interconnected 3D network to enhance a panel's performance and limiting the thickness of the core volume so that the panel can be kept relatively thin and light. In preferred embodiments the core volume is reinforced by 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15 layers of metal mesh, preferably 8 to 12 layers of metal mesh. Such preferred numbers of layers of metal mesh are suitable for 20-25 mm thickness panels. Of course the number of layers may be chosen depending on the material of the metal mesh, the size of wires forming the mesh, the thickness of the panel, the size and/or shape of openings in the mesh, as well as the performance requirements of a given panel.

It has surprisingly been found that panels comprising multiple layers of metal mesh reinforcement (preferably defining an interconnected network of 3D wire cells) in a core volume, impregnated and surrounded by cement mortar, display an improvement in fire and/or blast resistance as compared to currently available UHPFRC panels, while also being thinner and lighter. This is attributed to the elastic response provided by the compact core of metal mesh reinforcement (preferably defining an interconnected network of 3D wire cells) in combination with a surface covering outside the core volume to reduce the risk of explosive spelling and fragmentation under blast effect. Thus when viewed from a second aspect the present invention provides a concrete panel comprising a core volume of cement mortar reinforced by multiple layers of metal mesh, wherein the core volume is covered on all sides by surface layers of cement mortar containing metal fibres. The cement mortar may or may not contain polymer fibres.

The mesh layers are preferably joined together as mentioned above, and/or the number of layers may be chosen as discussed above. Thus in a preferred set of embodiments the multiple layers of metal mesh are fixedly connected or integrated together to define a network of 3D wire cells. It will be appreciated that the same effect(s) may be achieved even if the network of 3D cells is not formed by layers of metal wire mesh. Any suitable network of 3D wire cells, for example an integrally formed 3D cellular structure, may be used wherein the cells are formed by the metal wires of the 3D network.

When viewed from another aspect, the present invention may be considered to provide a cement mortar panel containing metal fibres and polymer fibres, wherein a core volume of the panel is reinforced by a network of 3D wire cells.

When viewed from yet another aspect, the present invention may be considered to provide a cement mortar panel comprising a core volume reinforced by a network of 3D wire cells, wherein the core volume is impregnated, and covered on at least two sides, by cement mortar containing metal fibres. The cement mortar may optionally contain polymer fibres as well.

There will now be described some preferred features according to any of the aspects of the invention outlined above, which may be taken alone or in combination with other of the described features.

Preferably the layers of metal mesh, where provided, are arranged in a staggered pattern. In other words, the grid openings in one layer are staggered relative to those in an adjacent layer. The network of 3D wire cells may comprise cells that are offset from one another. Using a staggered pattern has been found to significantly improve the strength and ductility of a panel as compared to standard reinforced concrete, providing optimum efficiency in resisting blast energy without failure, fragmentation or breaking down. The metal mesh layer(s) or cellular network provide a robust core for the panel to resist fire including explosive spalling and to prevent fragmentation under blast load. Staggering the layers may also help to make the core volume compact.

The metal mesh layers or network of 3D wire cells may have any suitable opening size. In preferred embodiments the mesh or network opening size may be selected from one or more of: 8 mm, 10 mm, 13 mm, 15 mm, 18 mm, 20 mm or 25 mm. The shape of the openings can be rectangular, square, hexagonal, etc. Based on test results obtained for 13 mm and 25 mm mesh size, a 16 mm opening size (for square shaped openings) is recommended for optimum performance. The metal mesh itself, or the wires forming the 3D cells in the network, may have a wire diameter of around 1 mm (e.g. gauge 19).

Another factor that can be important for a panel's performance is the quality of the concrete i.e. cement mortar casting. Preferably the steel mesh layers or network of 3D wire cells in the core volume are fully impregnated by the cement mortar. If the cement mortar does not penetrate through the openings in the mesh layers or 3D network then hollow pockets can be formed where moisture can accumulate with no concrete i.e. cement mortar material to resist explosion forces. One way of achieving good impregnation is to ensure that the cement mortar only contains fine aggregate material having a size less than 5 mm, and preferably contains only sand. In another example, one way of achieving good impregnation is to ensure the cement mortar has sufficient workability, preferably more than S4 (BS8500), by adding an appropriate amount of plasticizer. In preferred embodiments a plasticizer is added in an amount of 73 litre per m³ of mortar mix. Suitable plasticizers are based on polycarboxylate polymers or polycarboxylic ether (PCE) based polymers (commonly known as “superplasticizers”). A preferred plasticizer may act through a steric repulsion mechanism.

It has been found that the thickness of the core volume relative to the thickness of the panel can be important for ensuring that there is an optimal surface layer of cement mortar surrounding the core to protect the concrete panel from explosive spalling, to prevent fragmentation under blast load and to increase the ductility of the panel. In a preferred set of embodiments the core volume forms around 70-75% of the thickness of the panel. Preferably the core volume is centrally located so that it is covered by around 15% thickness of the panel formed of cement mortar on either side, preferably on all sides. For example, in a 25 mm thick panel there may be provided 12 layers of steel wire mesh (1 mm diameter; gauge 19) to fill a core volume having a thickness of 18-19 mm and covered by 3-3.5 mm of cement mortar. Alternatively, a 25 mm thick panel may be provided with 18-19 mm thickness of core volume reinforced by a network of 3D wire cells. Preferably the surface cover outside the core volume has a thickness not less than 3 mm and not greater than 10 mm. A surface layer thickness of around 4 mm may be preferred. If the cement mortar covering is too thick then steam generated in the core due to the heat of a fire may not be able to escape, resulting in explosion of the panel. In order to help ensure an even cover of the desired thickness, it is preferable that the metal mesh layers or network of 3D wire cells are securely positioned in a mould, e.g. by using spacers (such as steel bolts), to ensure that the cement mortar cover is achieved and maintained during casting.

According to the first aspect of the invention, and at least some embodiments of the other aspects of the invention, the cement mortar contains polymer fibres to enhance the explosive spalling resistance of the concrete. Polypropylene fibres may be used. In one preferred embodiment, the polypropylene fibres are of 12 mm length, 18 micron diameter and 160° C. melting temperature.

In a preferred set of embodiments the metal fibres are preferably steel fibres, and further preferably micro steel fibres. The micro steel fibres have a diameter <1 mm, for example a diameter of 0.2 mm. The steel fibres may be 10-20 mm in length, for example around 13 mm long. Using a hybrid of polymer and steel fibres to reinforce the cement mortar has been found to be particularly effective in increasing the mortar strength and preventing cracks from appearing. While the polymer fibres make the panels less susceptible to explosive spalling, the addition of steel fibres significantly reinforces the surface layers that normally suffer from spelling during fire and fragmentation under blast effect. A typical mix may contain around 1-3% or 1-4% (of mortar volume) of steel fibres and around 1-3 kg per M³ (of mortar volume) of polypropylene fibres.

The fibre content can be adjusted e.g. depending on the composition of the cement mortar and its strength. It may be preferred that the fibre content is not too high, so that the fibres do not unduly hinder the cement mortar in impregnating the mesh layers or network of 3D wire cells. However the fibre content may be higher than in known UHPFRC compositions. In one exemplary embodiment, the concrete i.e. cement mortar panel is made from 970 kg of dry cement, 156 kg of steel fibres and 1.5 kg of polypropylene fibres per m³ of panel. Thus the fibre content may be around 10-20%, e.g. around 15%, by weight as compared to the dry cement in the mix. Of course other ingredients may be added to the concrete i.e. cement mortar mix, as well as water, including sand, silica, GGBS and plasticizer.

The cement mortar preferably has a high strength, e.g. in the range of 90-200 MPa, to provide good resistance to compressive and tensile stresses. An exemplary cement mortar has a strength of 92 MPa. However, the main component that has been found to provide strength against blast and fire is the wire mesh reinforcement or other network of 3D wire cells in a core volume of the panel. The mesh or other cellular reinforcement shifts the panel's behaviour from the brittle zone to the ductile zone. It has been estimated that multiple staggered layers of steel wire mesh or a network of 3D wire cells can make the concrete i.e. cement mortar panels 10-12 times more ductile/elastic than similar panels (without mesh reinforcement or a wire network)-providing an improved performance in withstanding blasts.

As a result of the compact core volume of wire mesh reinforcement or network of 3D wire cells, a concrete i.e. cement mortar panel according to the present invention can be made thinner (and thus lighter) than conventional concrete panels including UHPFRC in order to withstand the same blast load. The concrete i.e. cement mortar panel is preferably 20-25 mm thick, most preferably 20 mm thick. The panel may be square or rectangular with sides measuring 1, 2 or up to 3 m long. An exemplary panel may be 2.5 m×1.25 m. Preferably the weight of the panel is relatively low, e.g. in the range of 38-50 kg/m² for a 25 mm thickness panel. A standard panel (0.5×0.5×0.02 m) may weigh around 9 kg (dependent on moisture content). This makes the panels quick and easy to install manually. The panels can be handled and fitted on site.

An advantage of the panel's construction is that it can be produced in a curved shape e.g. to fit curved structures including tunnels.

Tests have shown the following benefits of concrete i.e. cement mortar panels according to embodiments of the present invention.

Blast Resistance

-   1. Excellent blast/impact resistance. Tests have shown that the     0.5×0.5×0.025 mm panels can withstand an impact equivalent to a     blast of 800 kg TNT at 30 m standoff distance or equivalent and     complying with US Department of Defence UFC 3-340-02 blast response     criteria. This is far above the (EXV(10) ISO 16933:2007(E)) criteria     of blast resistance which is equivalent to 100 kg of TNT at 10 m     standoff distance. -   2. The slabs have approximately 10-12 times higher ductility than     similar slabs with no steel mesh layers or other 3D wire cells     network reinforcement making them ideal to withstand blast shock     wave.

Fire Resistance

-   1. Complete elimination of explosive spalling. -   2. The slabs are not combustible, i.e. they do not burn in fire     which makes them 100% safe in fire. This makes them superior to     other products in the market. -   3. Very good thermal insulation. After ½ hour of fire exposure the     temperature on the unexposed side of the panel remains around than     90° C. and around 200° C. after 1 hr of fire exposure when the     temperature on the exposed side is around 1000° C.

Concrete i.e. cement mortar panels according to embodiments of the invention can be made using standard techniques, with the additional step of forming a core volume of metal mesh layers or comprising a network of 3D wire cells. The core volume preferably consists of a bulk volume in the core of the cement mortar panel. The cement mortar containing the metal fibres (and, optionally, polymer fibres) is cast into and around the core volume of mesh layers or 3D network of wire cells to form a panel in a desired shape. A vibrating table may be used to aid impregnation of the mesh layers or 3D network of wire cells with the cement mortar.

According to a yet further aspect of the present invention there is provided a method of making concrete panels, comprising the steps of: securely positioning one or more metal mesh layers in a mould to form a core volume; preparing a cement mortar containing metal fibres (and optionally containing polymer fibres); and forming a panel by casting the cement mortar in and around the core volume so as to impregnate the metal mesh layers with mortar and surround the core volume with surface layer(s) of mortar. Any of the preferred features described above may optionally be achieved as part of the manufacturing process. For example, in one set of embodiments the method may further comprise forming the core volume by joining together multiple layers of metal mesh, preferably in a staggered arrangement.

As is mentioned above, in a preferred set of embodiments the method may comprise a step of fixedly connecting or integrated together multiple layers of metal mesh, before the positioning step, to provide a network of 3D cells to form the core volume. It will be appreciated that the benefits discussed above may be achieved even if the network of 3D cells is not formed by layers of wire mesh. Any suitable network of 3D wire cells, for example an integrally formed 3D cellular structure, may be used. Thus when viewed from another aspect, the present invention may be considered to provide a method of making cement mortar panels, comprising the steps of: securely positioning a network of 3D wire cells in a mould to form a core volume; preparing a cement mortar containing metal fibres (and optionally containing polymer fibres); and forming a panel by casting the cement mortar in and around the core volume so as to impregnate the network of 3D wire cells with cement mortar and surround the core volume with surface layer(s) of cement mortar.

Concrete i.e. cement mortar panels according to embodiments of the present invention may find a wide variety of uses, fitted as cladding to existing structures or used to build new structures. The panels described herein may find use in fire and/or blast protection. Alternatively, or additionally, the panels described herein may find use as a ground or road surface. In some embodiments there may be provided a method of surfacing or resurfacing a load-bearing surface such as a road, pavement, walkway, etc. using the panels described herein.

For blast protection, the panels described herein may find use as walls or wall coverings, claddings, linings, etc. The panels may be fitted on building facades, internal walls or ceilings which are considered most vulnerable during a fire and/or explosion. According to a preferred set of embodiments there is provided a surface protection system comprising one or more concrete i.e. cement mortar panels as described above, the panels being fitted in a supporting frame. The supporting frame makes it easier to fit the panels in place, especially when applying cladding to an existing structure. The frame may comprise a female connection arrangement designed to mate with the male profile of the panels. The male profile may be provided by the panel itself, but to avoid risk of damage the panel is preferably provided with a male frame, e.g. fitted to one face of the panel, that connects with the female frame. A “click to fit” system may be provided, e.g. wherein the female connection comprises snap-fit means for fixedly connecting a panel to the frame, preferably in an irremovable manner. Advantageously, the “female” frame can be attached to an existing wall or other structure in advance and then the “male” panels can be fitted into the frame when they are ready to be installed. The panels may be manufactured on site and fitted one-by-one.

Such a system in considered novel and inventive in its own right, regardless of the type of concrete panels used, and thus when viewed from a further aspect the present invention provides a surface cladding system comprising a frame for attachment to a surface and snap-fit connection means arranged to receive a concrete cladding panel and connect it to the frame in use. Preferably the panel is irremovably connected to the frame by the snap-fit connection means. The snap-fit connection means may be provided on the frame or on a separate member, for example a member that can be fitted or attached to a panel. Such a system enables a surface to be clad with concrete panels more quickly and simply than if the panels were attached using conventional means such as bolts. However, the panels can also be fitted to the frame using these conventional steel bolts where desired. The concrete panels may be cement mortar panels as described above.

In designing a surface cladding system that is particularly suitable for providing blast resistance, the Applicant has recognised that the frame connection can itself be used to help absorb the shock of an explosion. Thus in a preferred set of embodiments the frame and its connection means and/or the panels are provided with one or more shock absorbing means, for example in the form of fuse bolts. Such a system has improved blast resistance and may be considered novel and inventive in its own right. Thus when viewed from a yet further aspect the present invention provides a surface cladding system comprising a frame for attachment to a surface, the frame arranged to receive one or more concrete cladding panels in use, and further comprising shock absorbing means arranged on or between the panel(s) and the frame. The shock absorbing means is preferably ductile, thereby absorbing blast impact by bending. The shock absorbing means may be in the form of fuse bolts. One or more metal bolts may be used as shock absorbing means. The shock absorbing means may be provided on the frame, and/or arranged between the frame and panels, and/or provided on the panels.

Some preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

FIG. 1 shows a concrete i.e. cement mortar panel according to an embodiment of the invention;

FIG. 2 is a cross-sectional view of the panel of FIG. 1;

FIG. 3 shows a cement mortar panel according to another embodiment of the invention;

FIG. 4 is a cross-sectional view of the panel of FIG. 2;

FIG. 5 is a schematic diagram of a cladding system;

FIG. 6 shows some details of such a cladding system;

FIG. 7 shows slab 7 (top) and slab 8 (bottom), before fire test (A) and after fire test (B);

FIG. 8 shows slab 9 (bottom) and slab 10 (top) with polypropylene fibres, before fire test (A) and after fire test (B);

FIG. 9 shows control slab 15 (bottom) and slab 16 (top), before fire test (A) and after fire test (B), with the destruction of slab 15 due to explosive spalling under fire;

FIG. 10 shows the outstanding performance of panels in Group 1 under impact loads: (a) Slab 17 after 13 impacts, (b) Slab 19 after 11 impacts, (c) Slab 23 (control UHPFRC panel) after 2 impacts, (d) Slab 19 shown in cross-section;

FIG. 11 is a graph showing deflection of slabs in Group 1 during impact tests indicating the high ductility as compared with conventional UHPFRC slabs;

FIG. 12 shows the outstanding performance of panels in Group 2 under impact loads: (a) Slab 18 after 7 impacts, (b) Slab 20 after 5 impacts, (c) Slab 24 (control UHPFRC panel) after 2 impacts;

FIG. 13 is a graph showing deflection of slabs in Group 2 during impact tests indicating the high ductility as compared with conventional UHPFRC slabs;

FIG. 14 shows the capacity of the tested slabs to absorb the energy of impact compared with UHPFRC; and

FIG. 15 shows the central deflection of the slabs tested under static loading compared with UHPFRC slabs.

There is described with reference to the Figures a fire-blast protection system which aims to provide protection for buildings, tunnels, industrial premises (or any other structures) against events of: a) fire only, b) blast only (including terrorist attacks), c) fire and blast at the same time. The system is based on using 0.50×0.50×0.02 m or 2.5 m×1 m×0.025 m (or any other custom sizes) reinforced concrete i.e. cement mortar panels. The panels can also be produced in a curved shape to fit curved structures including tunnels. As is seen from the embodiment of FIG. 1, the panels are relatively thin with a 20 mm to 25 mm thickness which makes them logistically efficient (ease of handling fitting, etc.) The panels provide excellent resistance against blast forces. Tests have shown that the 0.5×0.5×0.025 m slabs can withstand an impact equivalent to blast of 800 kg TNT at 30 m standoff distance or equivalent and complying with US Department of Defence UFC 3-340-02 blast response criteria. That is far above the (EXV(10), ISO 16933:2007(E)) criteria of blast resistance which is equivalent to 100 kg of TNT at 10 meters standoff distance. The concrete i.e. cement mortar panels have a significant improvement in ductility and elasticity properties if compared to standard similar panels. The panels also provide 100% fire resistance as they are completely resistant to explosive spalling during fire. They are also not combustible i.e. they do not burn during fire. In addition, the panels have good thermal insulation capacity. That is the ability to keep the temperatures on the unexposed (to fire) side of the panel significantly lower than the exposed side.

The panels are not expensive to produce. The average cost to produce one m² of 20-25 mm thickness panels is £30-£40 (materials only). The panels have a reasonable weight to be handled and fitted on site. The average weight of a 0.5×0.5×0.020 m panel=9 kg (dependent on moisture content). The dual function of resisting fire and blast forces, the low cost, the small weight, and the small thickness make the system unique. The panels allows standard finishing (painting, plastering, rendering, etc) to be carried out to suit demands.

The panels seen in FIGS. 1 and 2 are fabricated using cement mortar of ultra-high strength 90-200 MPa and reinforced with a hybrid of polypropylene fibres (12 mm length, 18 micron diameter and 160° C. melting temperature) and micro steel fibres (13 mm length and 0.2 mm diameter, tensile strength 2000 MPa). The mix contents in one m³ are shown in Table 1 for two different examples. In one example, the mixing time is: 5 minutes for dry ingredients (cement+sand+silica fume)+2 minutes with water+2 minutes with steel fibres+2 minutes with polypropylene fibres+1 minute all materials. Total mixing time=12 minutes. In another example, the mixing time is: 4 minutes for dry ingredients (cement+sand+silica fume)+3 minutes with water+3 minutes with steel fibres+3 minutes with polypropylene fibres+2 minutes all materials. Total mixing time 15 minutes. Curing the slabs can be either in water with room temperature for a period of 3-28 days or more. The strength of the slabs can be increased by curing in hot water up to 90° C. for 3-7 days or more.

TABLE 1 Ingredients Quantity Unit Mix one Cement 970 kg Water 243 kg Sand 668 kg Silica fume 245 kg Superplasticizer (based on 73 litre polycarboxylate polymers) Steel fibres 156 kg Polypropylene fibres 1.5 kg Mix two cement 628 kg water 251 kg sand 680 kg silicafume 258 kg GGBS (Ground Granulated 330 kg Blast-furnace Slag) Superplasticizer (based on 72 litre polycarboxylate polymers) Steel fibres 79 kg Polypropylene fibres 0 kg

The main part of reinforcement of the panels is the galvanised steel wire mesh (gauge 19=1 mm wire diameter) which defines a 3D network of cells as shown in FIGS. 1, 2, 3 and 4. FIG. 2 shows the 3D network of cells formed from interconnected layers of galvanised wire mesh 2 staggered and packed together as one cage, with a cement mortar cover 4. The steel wire mesh 2 can be of different openings sizes between 13 mm to 25 mm. A 16 mm (square) opening size is recommended for optimum performance. For example for 25 mm thickness panels the steel wire mesh reinforcement 2 (or other network of 3D wire cells) should form around 75% of the thickness of the slab leaving 15% of mortar cover 4 (not to exceed 4 mm) from edges, top and bottom surfaces, as is shown in FIG. 2. The thickness of the steel wire core S_(p) is determined by the cover thickness C_(p) for a given panel depth d (see FIG. 2) The cover C_(p) should be 15% of the total panel depth d. In any case the cover should not be less than 3 mm and not greater than 10 mm. The steel wire mesh core thickness S_(p) is the remaining depth after deducting the cover C_(p) for both of the top and bottom sides of the panel (S_(p)=d−(2×C_(p))). In Table 2 below are some examples of the steel wire mesh and cover thickness.

TABLE 2 Panel Cover Steel core No. of 1 mm thickness thickness wire mesh diameter wire mesh (mm) each side (mm) thickness (mm) layers (gauge 19) 10 3 7 4 20 3 14 9 25 4 17 12 50 7.5 35 22 100 10 80 52 Tolerance ± 0.5 mm

The steel wire mesh layers 2 are grouped together and packed as one group (as shown in FIG. 2) by means of point welding or using steel wires. The openings of the layers are placed in staggered pattern as shown FIG. 1. That is, each steel layer staggers the next layer. Using the steel wire mesh in this pattern significantly enhances the strength of the panel and considerably increases the ductility and elasticity of the panels. It will also provide a robust core for the slab to resist fire including explosive spalling.

It is also seen from FIG. 1 that there is a light galvanised male frame 6 at the back of the panel (with 1 cm clearance from the edges).

In another embodiment seen in FIGS. 3 and 4, a cement mortar panel 1 is formed of ultra high strength cement mortar with polypropylene and steel fibres. The core 2 consists of a network of 3D wire cells. The core 2 may be an integrally formed cellular structure. The wire network ensures an interactive performance between the cells. It is seen from the cross-sectional view of FIG. 4 that the wire network in the core 2 is impregnated and covered by the cement mortar forming the surfaces of the panel 1.

The steel fibres are used to increase the mortar strength, increase the ductility and reduce cracks appearance. This will significantly reinforce the external layers that normally suffer from explosive spalling during fire and fragmentation under blast effect. The addition of polypropylene fibres makes the panels less susceptible to explosive spalling normally encountered in concrete under fire.

As is seen from FIGS. 5 and 6, the system can be assembled on site by fitting the “male” panels 1 into a “female” light frame 8 fitted on the existing wall 10 (that needs to be protected) e.g. using a “click to fit” technique. The female light steel frame 8 is fitted on the existing wall or ceiling using DIY bolts and plastic plugs. The “male” panels 1 are then fitted by clicking into the “female” steel profile. Other methods of fitting can be utilized depending on the blast risk assessment. During casting the panels 1 are fitted with light galvanized “male” steel frame 6 at the back face (leaving 1 cm from the edges of the panel). This frame 6 then clicks into the “female” frame 8 that has been formerly fitted on the existing wall 10 on site. The “male” plate 6 is designed to work as a “fuse” by bending in cases of excessive blast shock.

The system robustness against explosions can be enhanced by using “fuse” bolts 12 (that can be part of the “click to fit” mechanism) as supports of the panels 1. The fuse bolts 12 are designed to fail under high explosion impact relieving by that the blast pressure on the panel face. It is recommended that 6-12 bolts 12 are used on the perimeter of the rear face (away from blast) of the panel 1 depending on the panel's fitting procedure, as shown in FIG. 6. In case of high explosion load the bolts fail (see FIG. 6) under the impact impulse. A failed bolt 12′ is seen in close-up after the blast. This will absorb the blast impact energy and relieve the blast pressure on the panels and significantly increase the system blast resistance capacity. The bolts' diameter and quantity are determined to ensure that they fail under the impact load by bending. Table 3 below shows some diameters of the bolts for some blast intensities.

TABLE 3 Blast load intensity Bolts 500 kg of TNT @ 30 m standoff 10 bolts of 6 mm diameter 500 kg of TNT @ 40 m standoff 10 bolts of 4 mm diameter 500 kg of TNT @ 50 m standoff 10 bolts of 3 mm diameter

Experimental Results

26 panels measuring 500×500×25 mm were produced. The panels were made using a 92 MPa mortar having a high workability to allow casting into the openings of the grid mesh. Either 8 or 12 mild steel mesh layers were used for the core reinforcement. The mesh openings were either 13 mm or 25 mm. The mesh layers had to be compacted so as to be able to fit 12 layers of mesh into a thickness of 25 mm leaving 3 mm of concrete i.e. cement mortar cover only. Polypropylene fibres were optionally included in some of the panels.

Fire Tests

Fire tests were performed under the ISO 834 standard fire curve and prolonged for 60 minutes each to investigate mainly two parameters:

-   1. Explosive spalling of panels; and -   2. Thermal insulation capacity of the panels.     All the slabs were cured in water for 7 days to ensure extreme     moisture content condition in order to simulate the most extreme     situations in practice under which explosive spalling may take     place.     Panels without Polypropylene Fibres:

Test 1

This test involved two slabs 3 and 4. The mesh was 8 layers of 13 mm grid (square openings) of mild steel staggered. In this test slab 4 suffered from explosive spalling. However slab 3 did not suffer any explosion, but an edge piece of slab 3 was defragmented at minute 11 due to the explosion of slab 4. Examining the slabs after fire has shown that poor production quality increased the risk of spalling as the cover of slab 4 was around 12 mm while it was supposed to be 6 mm. This doubling of the design cover thickness led to steam not being able to escape hence explosion happened.

Test 2

This test involved two slabs 5 and 6 with 12 layers of 25 mm (square) opening size mesh. As expected the slabs performed superiorly and zero explosive spalling was recorded in this test. Just a small area was sloughed at the front surface of bottom slab 6 and that was due to the attachment of the thermocouple which created initial cracking (upon concrete i.e. cement mortar setting). Of course the test thermocouple would not exist in a real life situation.

Test 3

This test involved two slabs 7 and 8 with 8 layers of 25 mm mesh (square openings). In this test the slabs performed very well and no explosive spalling was recorded on the heated surface during the period of the test. FIG. 7 shows the panels before and after the fire test (NB. the thermocouple wire which can be seen should not be mistaken for a crack).

Panels with Polypropylene Fibres:

These panels contained polypropylene fibres to enhance the explosive spalling resistance. Six slabs were tested in this group.

Test 4

This test has marked a complete and full success of the panels' performance under fire. Slab 9 (8 layers, 13 mm mesh (square) opening, with 28 g of polypropylene fibres) and slab 10 (12 layers, 13 mm mesh opening, with 7 g of polypropylene fibres) were involved in the test. The slabs were completely unaffected and intact by the end of the one hour fire test, as is shown in FIG. 8. The slabs also showed very good thermal insulation capability as the temperature on the unexposed side of the slab was around 95° C. after half an hour and 200° C. after 1 hrs while the fire temperate was around 1000° C.

Test 5

This test confirmed the results obtained in Test 4 and indicated a complete success of the slabs in demonstrating full fire resistance. Slab 11 (8 layers, 13 mm mesh opening, with 7 g of polypropylene fibres) and slab 12 (12 layers, 13 mm mesh opening, with 27 gm of polypropylene fibres) were involved in the test. The slabs were completely unaffected and intact by the end of the one hour test.

Test 6

This test involved two slabs (13 and 14). Both slabs had 12 layers of 25 mm grid and both had 14 gm polypropylene fibres. The two slabs remained intact during the whole test with no explosive spalling occurring.

Test 7—Control

This was the last test and involved two control slabs 15 and 16 that did not have any steel mesh (or other 3D network of wire cells) or polypropylene fibres. However the slabs had the full strength of 92 MPa and were reinforced with 2% (in volume) of steel fibres which is the same amount in other panels that had steel wire mesh. The test was stopped after 13 minutes due to a very severe and loud explosion which completely tore slab 15 to parts and made continuation of the test unfeasible. The slab was completely destroyed by explosive spalling, as is shown in FIG. 9.

A summary of the fire test results is shown in Table 4.

TABLE 4 No. of Mesh Polypro- mesh opening pylene Fibres Specimen Test layers size (mm) (kg/m³) Explosions slab 1 12 13 0 N/A slab 2 12 13 0 N/A slab 3 1 8 13 0 no explosion slab 4 8 13 0 explosion slab 5 2 12 25 0 no explosion slab 6 12 25 0 no explosion slab 7 3 8 25 0 no explosion slab 8 8 25 0 no explosion slab 9 4 8 13 4 no explosion slab 10 12 13 1 no explosion slab 11 5 8 13 1 no explosion slab 12 12 13 1 no explosion slab 13 6 12 25 2 no explosion slab 14 12 25 2 no explosion slab 15 7 0 — 0 Very severe explosion slab 16 0 — 0 N/A

Blast/Impact Tests

Performing real blast tests requires special facilities and resources. Therefore, drop weight impact tests were performed to simulate blast effect and to give a primary assessment of the panels' resistance to explosions.

Conducting drop weight impact tests also requires expensive equipment and devices which were not available at the University of Ulster. Therefore, alternative test arrangements were adopted to fit within the project budget. The tests were performed by dropping a steel mass of 27.13 kg from two heights (1.5 m and 2 m) to simulate explosion energy. Six weight drop tests were performed in total. Two groups of slabs each consisting of three specimens were involved in the impact tests. Each group was subjected to impact energy by dropping a 27.13 kg mass from a calculated height to simulate the energy and pressure exerted by explosion. Table 5 shows details of the impact tests.

The tests showed an unprecedented performance of the slabs as compared with control slab 23 and 24 (with steel fibres only). The test showed a remarkable success in shifting the behaviour of concrete from the brittle zone to the ductile zone. This makes the panels very efficient to resist and absorb blast energy.

TABLE 5 Equivalent Drop Kinetic reflected Reflected Reinforcement Height Energy Equivalent blast pressure Time Impulse Slab ref. type (m) (J) load (MPa) (ms) (MPa · ms) Gro. 1 Slab 17 mesh 12 × 13 mm 1.52 m 404.5 500 kg of TNT 0.172 13.2 1.13 Slab 19 mesh 12 × 25 mm at 30 m standoff Slab 23 no mesh Gro. 2 Slab 18 mesh 12 × 13 mm 2.02 m 537.6 800 kg of TNT 0.262 10.80 1.41 Slab 20 mesh 12 × 25 mm at 30 m standoff Slab 24 no mesh

The panels showed an outstanding “spring” (ability to bounce back impact objects) indicating a high elasticity behaviour of the concrete i.e. cement mortar slabs which is important in blast shock absorption. The slabs showed an approximately 10 times bouncing/spring effect capacity if compared with normal control slabs. The slabs were observed to bounce back the 27 kg drop weight to a height of approximately 500 mm compared with 50 mm for the control slab. The tests also showed a high ductile deformability of the slabs compared with control slabs, making it ideal for blast absorption.

Group 1

Drop weight height 1.5 m.

Equivalent blast: 500 kg of TNT @ 30 m distance or well above EXV(10) of ISO 16933:2007(E).

The tested slabs performed remarkably and efficiently by resisting 13 impacts for slab 17 (12 layers of 13 mm mesh) and 11 impacts for slab 19 (12 layers of 25 mm mesh) with high strength capacity residual in the slabs after the tests. The control slab 23 which had no mesh but steel fibres failed in the first impact by deflection and was completely destroyed in the second impact. FIG. 10( a) to (c) shows the unique performance of the panels in terms of ductile deformability as compared to one of the control UHPFRC slabs. The slabs were observed to undergo a large deflection with no crushing after 11 impacts, as is seen in FIG. 10( d).

FIG. 11 is a graph showing deflection of slabs in Group 1 during impact tests indicating the high ductility as compared with conventional UHPFRC slabs. Line 1 relates to slab 23 (control, no mesh) and shows complete failure by fragmentation. Line 2 relates to slab 19 with 12×25 mm mesh. Line 3 relates to slab 17 with 12×13 mm mesh. FIG. 11 confirms the efficiency and the ductility of the panels compared with the control slab. FIG. 11 shows that slab 17 (13 mm mesh opening size) has shown higher stiffness than slab 19 (25 mm mesh opening size) while the control UHPFRC slab 23 showed very weak stiffness with a failure after first impact.

Group 2

Drop weight height 2 m.

Equivalent blast: 800 kg of TNT @ 30 m distance.

This group was subjected to blast/impact energy higher than group 1 as shown in Table 5 above. The slabs performed remarkably well: slab 18 (12 layers of 13 mm mesh) resisted 7 impacts, and slab 20 (12 layers of 25 mm mesh) resisted 5 impacts. The outstanding performance of the slabs can be compared with the control UHPFRC slab 24 which failed in the first impact and was completely shattered into pieces in the second impact. Pictures of the slabs after the impact tests are shown in FIG. 12. Development of deflections of slabs of Group 2 is shown in FIG. 13.

FIG. 13 is a graph showing deflection of slabs in Group 2 during impact tests. Line 1 relates to slab 24 (control, no mesh) and shows complete failure by fragmentation. Line 2 relates to slab 20 with 12×25 mm mesh. Line 3 relates to slab 18 (12×13 mm mesh).

An overall assessment of the absorption capacity of the cement mortar panels compared to UHPFRC is shown in FIG. 14. In FIG. 14, area 1 is slab 18 tested for energy absorption and area 2 is a comparison with UHPFRC.

Static Loading Tests

Three static tests were performed on 500×500×25 mm slabs. The tests involved applying a concentrated load at the central point of the surface of the 480×480 mm simply supported (on all four edges) slab. One slab contained a 3D network of wire cells formed from 12 layers of mesh with a 25 mm openings size. The second slab did not contain a 3D network of wire cells, while the third one was a plain cement mortar slab.

FIG. 15 shows the development of the central deflection of the panels with load increase for the three types of panels. The central deflection of the tested slab 26 (line 1) is compared to UHPFRC (line 2) and plain cement mortar (line 3). FIG. 15 clearly shows that the panels (line 1) demonstrated a spectacular ductility with a stress-deflection curve similar to steel behaviour. The outstanding performance of the slabs, exceeding the UHPFRC by around 2.2 times on load capacity and deformability, is shown in Table 6.

TABLE 6 Results of static loading tests Slab ref. Reinforcement type Max load KN Max deflection mm Slab 26 3D network of wire 26.82 47.03 cells 12 × 25 Slab 27 UHPFRC 12.88 21.16 Slab 28 Plain cement mortar 2.27 0.46 

1. A cement mortar panel containing metal fibres and polymer fibres, wherein a core volume of the panel is reinforced by a network of 3D wire cells.
 2. A panel as claimed in claim 1, wherein the network of 3D wire cells is defined by multiple layers of metal mesh that are fixedly connected or integrated together. 3-5. (canceled)
 6. A panel as claimed in claim 1, wherein the core volume is surrounded, and impregnated, by the cement mortar.
 7. A cement mortar panel comprising a core volume reinforced by a network of 3D wire cells, wherein the core volume is impregnated, and covered on at least two sides, by cement mortar containing metal fibres. 8-9. (canceled)
 10. A panel as claimed in claim 7, wherein the cement mortar also contains polymer fibres.
 11. A panel as claimed in claim 1, wherein the polymer fibres are polypropylene fibres. 12-14. (canceled)
 15. A panel as claimed in claim 7, wherein the network of 3D wire cells is arranged in a staggered pattern.
 16. A panel as claimed in claim 7, wherein the network of 3D wire cells has an opening size selected from one or more of: 8 mm, 10 mm, 13 mm, 15 mm, 18 mm, 20 mm or 25 mm. 17-18. (canceled)
 19. A panel as claimed in claim 7, wherein the network of 3D wire cells has a wire diameter of around 1 mm.
 20. A panel as claimed in claim 7, wherein the network of 3D wire cells in the core volume fully impregnated by the cement mortar.
 21. (canceled)
 22. A panel as claimed in claim 7, wherein the core volume forms around 70-75% of the thickness of the panel. 23-25. (canceled)
 26. A panel as claimed in claim 1, wherein the metal fibres are steel fibres.
 27. (canceled)
 28. A panel as claimed in claim 26, wherein the steel fibres have a diameter <1 mm.
 29. A panel as claimed in claim 7, wherein the cement mortar has a strength in the range of 90-200 MPa. 30-32. (canceled)
 33. A panel as claimed in claim 1, for use in fire and/or blast protection.
 34. A panel as claimed in claim 1, for use as a ground or road surface.
 35. A method of making cement mortar panels, comprising the steps of: securely positioning a network of 3D wire cells in a mould to form a core volume; preparing a cement mortar containing metal fibres (and optionally containing polymer fibres); and forming a panel by casting the cement mortar in and around the core volume so as to impregnate the network of 3D wire cells with cement mortar and surround the core volume with surface layer(s) of cement mortar. 36-37. (canceled)
 38. A panel made according to the method of claim
 35. 39-55. (canceled)
 56. A panel as claimed in claim 7 employed in fire and/or blast protection.
 57. A panel as claimed in claim 7 employed as a ground or road surface. 